The present invention relates to an ion beam generation apparatus used, for example, in an ion implanter system for implanting ions from an ion beam target into substrates such as semi-conductor wafers.
Ion implantation techniques, e.g. for modifying the electrical conductivity properties of semi-conductor materials, are known in the manufacture of integrated circuit structures in semi-conductor wafers. Such ion implanters generally comprise an ion beam generation apparatus having a source of ions of the element to be implanted in the semi-conductor wafer, and an extraction assembly for extracting ions from the source and forming a beam of the extracted ions. The ion beam so produced is then passed through a mass analyser and selector for selecting a particular species of ions in the ion beam for onward transmission for implantation in the wafer or target substrate.
The extraction assemblies used are conventionally triode extraction assemblies, so called because they involve an arrangement of three electrodes. A triode assembly requires mechanical adjustment of the electrodes to be made in order to optimise or xe2x80x9ctunexe2x80x9d the ion source for maximum beam current on the target substrate. In an attempt to simplify this xe2x80x9ctuningxe2x80x9d operation, it has been proposed to use a tetrode assembly having four electrodes. Such an assembly is disclosed in an article entitled xe2x80x9cBeam Steering in Tetrode Extraction Systemsxe2x80x9d (A. J. T. Holmes and E.Thompson published by the American Institute of Physics in 1981). A more recent tetrode assembly is disclosed in WO99/23685.
The tetrode assembly has four electrodes, each having at least one aperture to allow the passage of the ion beam. The first electrode is a source electrode which generally forms one wall of the ion source and is at the same potential as the ion source. The second electrode immediately adjacent to the first electrode is an extraction electrode which is set at a potential to attract ions out of the ion source. The third electrode is a suppression electrode which operates to prevent electrodes in the ion beam downstream of the ground electrode from being drawn into the ion source. The fourth electrode downstream of the suppression electrode is a ground electrode which restricts the penetration of the electric fields between the ground electrode and the ion source into the region downstream of the ground electrode.
The advantage of a tetrode structure is that the potential between the arc chamber of the ion source and the extraction electrode can be set independently of the potential between the ion source and the ground electrode. In this way, the energy of the ion beam emerging from the extraction assembly can be determined independently of the potential at which the ions are initially extracted from the arc chamber. This permits the extraction efficiency of the ion source to be optimised and simplifies the xe2x80x9ctuningxe2x80x9d of the ion source for maximum beam currents.
Although tetrode structures offer this potential improvement, they have not found wide acceptance in ion beam generation. To date, tuning of the tetrode assembly for a particular beam energy has been achieved by varying the voltage on each electrode. This works satisfactorily for medium energy beams. However, for high energy beams, the large potential between the extraction and suppression electrodes tends to cause breakdown between these electrodes. On the other hand, at low energies, the provision of the fourth electrode can be counter productive, as the overall length of the assembly is increased, and space charge repulsion effects cause unacceptable divergence of the beam after the extraction electrode with a consequential loss of beam current at the suppression electrode.
One approach to preventing arc discharge at higher beam energies is disclosed in the document entitled xe2x80x9cThree-Stage Acceleration System for High Energy Implanterxe2x80x9d (B. O. Pedersen and R. B. Liebert published in Nuclear Instruments and Methods in Physics Research B6 (1985) pages 258-263). In this approach, a further electrode, termed the acceleration electrode is positioned downstream of the extraction electrode to provide an intermediate potential level between the second electrode and the ground electrode. This results in a pentode system, namely one having five electrodes. Although this is beneficial in suppressing arc discharge, it will inevitably lengthen the extraction assembly, thereby worsening the problem of ion beam expansion due to space charge repulsion for low energy, high current beams. This arrangement is therefore equally incapable of providing an apparatus that allows maximum beam currents to be achieved over a wide energy range.
According to the present invention, there is provided an ion beam generation apparatus comprising an ion source for generating ions, and a tetrode extraction assembly comprising four electrodes for extracting and accelerating ions from the ion source, the extraction assembly comprising a source electrode at the potential of the ion source, an extraction electrode adjacent to the source electrode to extract ions from the ion source, a ground electrode, and a suppression electrode between the extraction electrode and the ground electrode, each electrode having an aperture to allow the ion beam to pass therethrough, wherein the gap between the extraction and suppression electrodes is variable in the direction of ion beam travel.
With this arrangement, the size of the gap between the extraction and suppression electrodes can be increased for high energy beams and decreased for low energy beams. Thus, the ability of the extraction and suppression electrodes to stand off the electric field without arc discharges occurring is enhanced allowing the apparatus to be used at maximum beam current to higher energy levels. On the other hand, at low beam energies, the gap between the extraction and suppression electrodes can be reduced, thereby reducing the effect of space charge repulsion.
The invention therefore provides an ion beam generation apparatus which increases the maximum beam currents that can be achieved over a wider energy range (typically 0.5-80 keV).
Further, as changing the gap between the extraction and suppression electrodes alters the focussing effect of the electric field, the invention allows better control of the beam shape over a range of beam energies.
The extraction field between the extraction and source electrodes is preferably controlled by varying the voltage alone. This allows the extraction electrode to be fixed with respect to the source electrode. This is a significant advantage of the tetrode, as it is important for the repeatability of beam tuning that the extraction and source electrodes be precisely aligned. Generally, each electrode is independently mounted to the apparatus housing through a suitable bushing which allows dimensional tolerances to build up between the source and extraction electrode making precise alignment difficult. If the extraction electrode is mounted directly to the ion source, the alignment between the two electrodes can be far more precise. The mounting of the extraction electrode on the ion source should be done through insulators which are shielded and cooled to prevent contamination of the insulator surface which can cause electrical breakdown.
The suppression and ground electrodes can be fixed with respect to one another, thereby allowing them to be mounted on a common structure. On the other hand, if greater flexibility is required, the suppression and ground electrodes may be mounted so as to be movable independently of one another.
The aperture in each electrode is generally an elongate slot. Preferably, the suppression and ground electrodes are movable relatively to the source and extraction electrodes in a lateral direction perpendicular to the beam direction and perpendicular to the lengthwise dimension of the slot. This provides additional control of the steering of the beam into the subsequent components of the apparatus.
Further, this can be used to compensate for any deflections of the beam caused by fringing magnetic fields (notably from the source magnet or analyser magnets), as well as matching the beam lateral position into the optimum region of the analyser magnet poles. This movement allows the beam strike on the electrodes to be reduced, thereby achieving higher beam currents, and also providing better control of the beam position. Preferably, the source and extraction electrodes are fixed, while the suppression and ground electrodes are laterally movable.
With the elongate slot, there is a tendency for space-charge expansion to cause the beam to blow up in the direction of elongation of the slot. This causes increased electrode strike, and hence a loss of beam current. In order to overcome this problem, at least one of the electrodes is preferably concave facing away from the ion source in the plane containing the direction of beam travel, and the direction in which the slot is elongate. Preferably, the concave electrode is the extraction electrode. This curvature focuses the beam down as it passes through the extraction electrode and into the analyser magnet. The degree of curvature is preferably such that it counteracts the space-charge expansion of the beam in this plane. The source electrode may be concave in addition to the extraction electrode.